1. Field of the Invention
The present invention relates to a biomagnetic field measurement apparatus for detecting a magnetic field generating mainly from the heart of an unborn child.
2. Description of the Related Art
For a magnetocardiograph that measures a magnetic field generating from the heart (heart magnetic field), it is necessary to bring a sensor close to the measurement portion of an examinee (the chest if the examinee is a grown-up person, the lower abdomen of mother's body if it is an unborn child). In the related art, the measurement position of sensor has been adjusted by vertically moving a cylindrical cryostat that holds the sensor or a bed on which the examinee lies. A problem with this method is that the signal intensity decreases because the sensor surface parallel with the horizontal plane is not necessarily parallel with the body surface. Especially when the heart magnetic field of unborn child is detected, it is necessary to bring the sensor surface close to the lower abdomen of mother's body, so that exact positioning is difficult to do.
To solve this problem, for example, in Patent Documents 1 to 6, ingenuity has been exercised in facilitating the positioning work by means of a mechanism for tilting the cryostat or a mechanism for moving the cryostat up and down that is added to a gantry.
Conventionally, a biomagnetic field measurement apparatus used for heart magnetic field measurement and brain magnetic field measurement has employed a method in which magnetic signals of a living body, which is a subject, are detected by a detector coil consisting of superconductive wiring and are transmitted to a superconducting quantum interference device (hereinafter abbreviated as SQUID). The detector coil plays a role in removing noise caused by the environmental magnetic field and thereby enhancing the signal-to-noise ratio (S/N ratio). The biomagnetism measurement and the detector coil have been explained in detail in Non-Patent Document 1.    [Patent Document 1] JP-A-7-265276    [Patent Document 2] JP-A-2000-139864    [Patent Document 3] JP-A-5-212008    [Patent Document 4] JP-A-6-197879    [Patent Document 5] JP-A-6-197880    [Patent Document 6] JP-A-9-327444    [Non-Patent Document 1] S. J. Williamson and L. Kaufman, Journal of Magnetism and Magnetic Materials, 22 (1981), 129-201
In the related art, the measurement position of sensor has been adjusted by vertically moving the cryostat that holds the sensor or the bed on which the examinee lies. This method has a problem in that the signal intensity decreases because the sensor surface is not necessarily parallel with the body surface. Also, even if the mechanism for tilting the cryostat or the mechanism for moving the cryostat up and down is added to the gantry, the cryostat must be moved up and down while being tilted. In this case, there arises a problem in that when the heart magnetic field of unborn child is measured, a sense of oppression is given to the examinee and the positioning work is difficult to do.
Also, as shown in Non-Patent Document 1 (FIG. 5), the conventional differential magnetism detector coil has been configured merely so that a magnetic field differentiated in certain one direction is detected. This method has a problem in that the environmental magnetic field is not reduced sufficiently in the case where the environmental magnetic field is strong, for example, in a magnetism shieldless environment. In order to reduce the environmental magnetic field, a method can be used in which the order of differential magnetism detector coil is increased. However, this method has a problem in that although the environmental magnetic field is reduced, the magnetic signals to be detected are also decreased.